Circuit breakers employing electronic trip units are available having a wide range of overcurrent pickup and time delay adjustments. Both analog and digital processors are capable of providing adequate circuit protection over a wide range of operating currents. For purposes of this disclosure, an "analog processor" continuously evaluates the input current and initiates a timing mode upon the occasion of an overcurrent condition. A good example of an analog timing circuit is a resistance-capacitance circuit (RC) wherein the RC time constant varies inversely with current. A "digital processor" evaluates only one value of the input current at one point in time. A good example of a digital timing circuit is the overflow time of a register within a microprocessor. To date, circuit breakers using electronic trip units have been limited to industrial type applications of higher ampere-rated frame sizes. The trip units are generally mounted on a front panel of the circuit breakers within a separate housing from the interruption mechanism and the current sensing transformers. However, with thermal-magnetic molded case circuit breakers wherein the operating mechanism and the thermal-magnetic trip unit are both contained within a common case, space requirements have heretofore limited the ratings of these circuit breakers to less than 1200 amperes.
The application of large scale integrated circuit techniques (LSI) to miniaturize the electronic trip unit components has substantially reduced the overall physical size of the electronic trip unit. The discrete circuit components, such as the current sensing transformers, timing capacitors and adjustable resistors, however, limit the extent to which the size of traditional electronic trip units can be reduced.
The purpose of this invention is to describe a circuit arrangement whereby the variable resistors used to set the trip current and time parameters for the circuit breaker electronic signal processor are reduced in number thereby allowing the printed wiring board containing the signal processor to become correspondingly reduced in size.